x² - 4xy - 2y² - 6 = 0 The angle of rotationof the graph Ax² + Bxy + Cy² + Dx + Ey + F = 0 necessary to transform it into an equation in x' and y' which contains no term in x'y' is calculated by or 45° or if A = C Your equation is x² - 4xy - 2y² - 6 = 0 In this case A=1, B=-4, C=-2, D=0, E=0, F = -6 Substituting in Choosing the smallest positive angle: or or Edwin